Articulated hydraulic manipulators are widely used for moving heavy loads. Commercial manipulators are most often equipped with a rotating load-grasping tool connected at the end of the manipulator via a pair of passive (unactuated) revolute joints. In free-space motion, these passive joints are subject to swaying motions due to the manipulator tip accelerations. Because these passive joints are not directly controllable due to their passive nature, a skilled driver is needed to compensate for the load swaying. In this paper, we extend the nonlinear model-based Virtual Decomposition Control (VDC) theory to cover anti-sway control of underactuated multiple degrees-of-freedom (DOF) hydraulic redundant manipulators. The proposed nonlinear controller performs the control design and stability analysis of the hydraulic robotic manipulator at the subsystem level. Experiments are conducted in a full-scale loader manipulator to verify that the proposed controller can efficiently damp the load swaying in a case study of redundant vertical plane motion.

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